1. Field of the Invention
The present invention relates to an echo canceller, and more particularly to an echo canceller applicable to, for example, a telephone apparatus for use in conference telephone or hands-free telephone.
2. Description of the Background Art
Conventionally, the echo canceller has its basis of operation on the assumption that an echo path it can estimate is linear time-invariant. Additionally, the echo canceller is adapted to proceed to convergence, when observing only an echo and a signal in question causing the echo, to thereby allow an actual echo path to be estimated to cancel the echo.
However, in a practical environment in which an apparatus equipped with the echo canceller is placed, a double talk may often occur in which a near end and a far end talker speak at the same time, background noise may be caused at a near end, or particularly with an acoustic echo canceller an echo path may often change. Therefore, adaptation for the echo canceller without any countermeasures thereagainst may cause its adaptive filter to diverge so as to cause howling.
The echo canceller is required to have not only theoretical aspects such as a rate of convergence and accuracy of estimation by advanced algorithm but also countermeasures on practical side for ensuring the robust stability in speech quality even under such practical conditions.
Typically, for example, as disclosed in Japanese Patent No. 2105375, the power of a received signal and an output signal of the echo canceller is used to detect a double-talking and a silent section.
However, such solutions based on detecting a change in power cannot distinguish an increase in signal power of the near end talker or power of near end background noise from an increase in echo power caused by a change in echo path, as is problematic. This problem leads to significantly serious disadvantage.
The reason for this is that the adaptive filter in the echo canceller needs to stop updating its coefficient in the case of double talk or an increase in near end background noise, whereas the adaptive filter has to accelerate update of the coefficient in the case of a variation in echo path. In other words, although both cases appear as the same phenomenon where the signal power (on an input or an output of an echo cancel adder) increases, both cases have to be controlled in the ways opposite to each other. Therefore, erroneous control over inappropriate one of these cases would lead to fatally deteriorating communication quality.
On the other hand, background noise on the near end talker side may often be treated from a viewpoint of improving naturalness of an auditory feeling of the far end talker rather than performance of the echo canceller. That type of solutions is exemplified by Japanese Patent Laid-Open Publication No. 2007-52150 and U.S. Pat. No. 6,181,753 to Takada et al., in which an echo canceller eliminates an echo and thereafter cancels a noise cancel from the signal. These solutions disclosed in the Japanese '150 Publication and Takada et al., cancel a noise after eliminating an echo because of the fact that the noise cancelling may often be nonlinear processing on the signal. That is also resultant from preventing the echo canceller from deteriorating in performance otherwise caused by nonlinearly of the echo path.
However, since these solutions do not contribute to the improvement of performance of the echo canceller, they are directly affected by the deterioration in performance of the echo canceller canceling the echo.
Moreover, disadvantageously, in the case of the background noise mixed with the echo as described above, the echo canceller can only eliminate the echo to the extent of the level of the near end noise at most. Such mutual reaction between both echo canceller and noise canceller may often result in failing to sufficiently attain the performance of thereof.
One typical solution for preventing performance of the echo canceller from deteriorating due to the near end noise is known as decreasing a step gain of the adaptive filter.
The step gain is a parameter for controlling the rate of convergence of the echo canceller. When the step gain is larger, the echo canceller converges more rapidly but is more sensitive to noise. When the step gain is smaller, the echo canceller can more effectively be prevented from being affected by noise but converges more slowly.
Japanese Patent Laid-Open Publication No. 237174/1996 discloses a solution for increasing a step gain at the start of operation of the device and gradually decreasing the step gain for a predetermined period from the start of the operation to thereby prevent an effect on a convergence period and noise to obtain the accuracy of the convergence.
However, since the control for decreasing the step gain is not always accurately related to the state of convergence of the echo canceller, the step gain may decrease even though the adaptive filter does not yet sufficiently converge. That may cause the convergence of the adaptive filter to interrupt.
In contrast to this, superior solutions for reflecting even the state of operation environment in which the echo canceller is placed to control a step gain to thereby facilitate the convergence of an adaptive filter are disclosed in Ryuichi Oka, et al., “A Method Steadily Reducing Acoustic Echo against Double-Talk” IEICE (The Institute of Electronics, Information and Communication Engineers) Tech. Rep., vol. 108, no. 69, EA2008-21, pp. 19-26, May 2008, and Japanese Patent Laid-Open Publication No. 2008-312199 and U.S. Patent Application Publication No. US 2007/0041575 A1 to Alves et al.
However, the solutions described in Ryuichi Oka, et al., and the Japanese '199 Publication and Alves et al., use a sub adaptive filter in order to distinguish an increase in power of a transmitted signal due to a variation in echo path and an increase in transmitted signal due to an increase in background noise or a double-talk signal, thereby estimating the power of near end noise.
The sub adaptive filter for use in the solutions described in Ryuichi Oka, et al., and the Japanese '199 Publication and Alves et al., is a sort of small echo canceller, which has the same essential problem as an adaptive filter. Ultimately as a result, the control of the step gain by the echo canceller significantly depends on the performance of the sub adaptive filter. That may significantly deteriorate the performance depending on acoustic background at a near end.
Particularly, these sub adaptive filters need a premise that the near end noise is not correlated with a far end signal, but this premise is not true in, for example, an office environment including an audio signal having the near end noise in itself significantly correlated with a far end signal, thereby causing a large error in output of the sub adaptive filter.
As a result, due to an inappropriate estimation on the effect of the near end noise signal, the step gain is rendered more excessive than necessary to cause the adaptation operation of the echo canceller to be unnecessarily sensitive to increase an error in entire output of the echo canceller. Thus, it is extremely difficult to prevent the performance from deteriorating due to the effect of the near end noise and a variation in echo path.